''Remember the chaos? Remember the waste and the bloated spending and the corruption that was plaguing the Olympics at that time?'' Mr Ryan asked sweaty throngs of supporters in High Point, North Carolina. ''Who did they turn to? Who did they ask to drop everything in his life and save the Olympics? That was this man right here.''

Mr Romney smiled and nodded in approval. On stage, the relaxed Mr Ryan exudes the kind of vigour and easy confidence that the more stiff and seemingly rehearsed presidential candidate has struggled to project.

''This is awesome,'' the 42-year-old congressman from Wisconsin exhorted as he looked out at the roaring crowd. When Mr Romney pledged to get rid of ''Obamacare'', Mr Ryan threw his arms in the air in approval, as if his beloved Green Bay Packers had scored a touchdown.

Mr Romney told the crowd: ''I am so happy to have my teammate now - the two of us.''

But after Sunday night's ''homecoming'' rally for Mr Ryan in Waukesha, the two men are not expected to appear together again for 2½ weeks, when they will reunite at the Republican National Convention in Tampa.

So, with the short honeymoon over, will Mr Romney go back to being the candidate he was, or will he carry this newfound momentum through the rest of the campaign? And as Mr Ryan begins his solo campaign swing, will his star power among conservative activists outshine Mr Romney's, drawing parallels with Sarah Palin, the Republican vice-presidential nominee in 2008?

Although Mr Ryan's validation of Mr Romney has gone over well, it's a tall order to seed a new narrative to prop up the candidate, whose likeability ratings have fallen under the onslaught of the Obama campaign's attacks on his personal finances and business ethics. And for all the attention Mr Ryan has received since Saturday, it's Mr Romney who will be debating President Barack Obama and whose name will appear at the top of ballots in November.

Despite the easy rapport between Mr Ryan and Mr Romney, there are emerging flashpoints, such as Mr Ryan's controversial proposal to slash spending and overhaul Medicare.

Both candidates have conspicuously avoided talking about the specifics in Mr Ryan's budget.

Democrats seized on the Medicare proposal in particular to paint the Romney-Ryan ticket as dangerous. Mr Obama called Mr Ryan a ''decent man'' who had a vision for the country ''that I fundamentally disagree with''.

A senior strategist for the Obama campaign, David Axelrod, told CNN: ''Congressman Ryan is a right-wing ideologue … He is quite extreme - good, good person, you know, genial person - but his views are quite harsh.''